U.S. patent application number 13/083163 was filed with the patent office on 2012-02-09 for image forming apparatus which corrects main scanning misregistration.
Invention is credited to Shukuya Yuuichiroh.
Application Number | 20120033267 13/083163 |
Document ID | / |
Family ID | 39348651 |
Filed Date | 2012-02-09 |
United States Patent
Application |
20120033267 |
Kind Code |
A1 |
Yuuichiroh; Shukuya |
February 9, 2012 |
IMAGE FORMING APPARATUS WHICH CORRECTS MAIN SCANNING
MISREGISTRATION
Abstract
An image forming apparatus includes a light source which
illuminates in response to image data, and a deflector to deflect
an optical beam output from the light source into a scanning beam
running along a main scanning line across an image forming area in
a main scanning direction. There are plurality of beam detectors to
detect the scanning beam at a plurality of different positions
along the main scanning line, the plurality of beam detectors
including first and second beam detectors detecting the scanning
beam at positions in front-end and rear-end sides, respectively, in
the main scanning direction.
Inventors: |
Yuuichiroh; Shukuya;
(Kunitachi-shi, JP) |
Family ID: |
39348651 |
Appl. No.: |
13/083163 |
Filed: |
April 8, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11857505 |
Sep 19, 2007 |
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13083163 |
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Current U.S.
Class: |
358/474 |
Current CPC
Class: |
B41J 2/473 20130101;
G02B 26/123 20130101; H04N 1/00477 20130101; H04N 2201/0072
20130101 |
Class at
Publication: |
358/474 |
International
Class: |
H04N 1/04 20060101
H04N001/04 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 19, 2006 |
JP |
2006-252433 |
Claims
1. An image forming apparatus, comprising: a light source
configured to illuminate in response to image data; a deflector
configured to deflect an optical beam output from the light source
into a scanning beam running along a main scanning line across an
image forming area in a main scanning direction; a plurality of
beam detectors to detect the scanning beam at a plurality of
corresponding positions along the main scanning line, the plurality
of beam detectors including first and second beam detectors
detecting the scanning beam at positions in front-end and rear-end
sides, respectively, in the main scanning direction; a clock
generator to generate writing clock signals for controlling an
illumination of the light source; a phase variable control unit
configured to vary a phase of the writing clock signals generated
by the clock generator, in units of 1/n of a writing clock cycle,
where n is an integer having a value of 2 or greater; a measurement
mechanism to measure a scanning period between detections of the
scanning beam by the first and second beam detectors; a frequency
corrector to correct a frequency of the writing clock signals so
the count number measured by the measurement mechanism becomes
substantially equivalent to a predetermined reference count number;
and a scanning period corrector to correct a scanning period at a
scanning area where phase variable control is not available.
2. The image forming apparatus is as claimed in claim 1, wherein:
the scanning period corrector stores phase variable amount data for
the scanning area where phase variable control is not available and
corrects the scanning period based on the phase variable amount
data at the scanning area where phase variable control is not
available.
3. The image forming apparatus is as claimed in claim 1, wherein:
the scanning period corrector is configured to calculate a phase
variable amount at the scanning area where phase variable control
is not available based on a phase variable amount at the scanning
area where phase variable control is available, and is configured
to correct scanning period based on the calculated phase variable
amount at the scanning area where phase variable control is not
available.
4. The image forming apparatus is as claimed in claim 1, the
scanning period corrector is configured to correct the scanning
period based on a frequency at an area where phase variable control
is available and based on a frequency at an area where phase
variable control is not available.
5. A method of controlling an image forming process, comprising the
steps: scanning a scanning beam along a main scanning line across
an image forming area in a main scanning direction using a clock
signal; measuring a scanning period between a first beam detector
and a second beam detector; correcting a frequency of the clock
signal so that the scanning period which has been measured becomes
substantially equivalent to a predetermined reference count number;
and correcting a scanning period at a scanning area where phase
variable control is unavailable.
6. A method according to claim 5, wherein the correcting of the
scanning period comprises: storing phase variable amount data for
the scanning area where phase control is not available, and
correcting the scanning period based on the phase variable amount
data at the scanning area where phase variable control is not
available.
7. A method according to claim 5, wherein the correcting of the
scanning period comprises: calculating a phase variable amount at
the scanning area where phase variable control is not available
based on a phase variable amount at the scanning area where phase
variable control is available, and correcting the scanning period
based on the calculated phase variable amount at the scanning area
where phase variable control is not available.
8. A method according to claim 5, wherein the correcting of the
scanning period comprises: correcting the scanning period based on
a frequency at an area where phase control is available and based
on a frequency at an area where phase control is not available.
Description
CROSS-REFERENCES TO RELATED APPLICATIONS
[0001] This application is a continuation application of U.S.
application Ser. No. 11/857,505, filed Sep. 19, 2007, which claims
priority to Japanese Patent Application No. 2006-252433, filed Sep.
19, 2006, the entire contents of which are incorporated herein by
reference. This application is related to U.S. patent application
Ser. No. 11/586,565 filed Oct. 26, 2006 and entitled "Image forming
apparatus capable of effectively correcting main scanning
misregistration", which is incorporated herein by reference. The
present invention may utilize any feature or technique set forth in
application Ser. No. 11/586,565.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention generally relates to an optical
apparatus and an image forming apparatus comprises the optical
apparatus.
[0004] 2. Discussion of the Background
[0005] A related art image forming apparatus such as a copying
machine and a printer, for example, is generally provided with an
optical device to produce an optical beam and uses it to write
image information on a photoconductor. Such a related art image
forming apparatus may employ a plastic lens to provide the optical
beam to meet recent trends of reducing costs and weights, for
example. In addition, the related art image forming apparatus has
increasingly penetrated its market and expanded the range of uses
and, as a consequence, it needs to satisfy the market demands for a
higher accuracy of an image magnification in a variable
magnification operation.
[0006] The related art image forming apparatus is generally
configured to modulate the optical beam based on image data and to
deflect the optical beam in a main scanning direction by a
deflection mechanism (e.g., a polygon mirror) so as to make the
optical beam scan a photoconductor surface through optical elements
including an F.theta. lens.
[0007] However, each of the related art image forming apparatuses
may have a different main scanning magnification due to a variation
in the optical device and/or properties of optical elements such as
the F.theta.. lens. A defective operation, for example, a
magnification error or a misregistration of a writing start
position, may also be generated by a change in a refractive index
or a shape of the plastic lens and a change in a scanning position
on the photoconductor caused by a variation in an environmental
temperature or a variation in a temperature of the apparatus
therein. Thereby, an image with a high quality may not be provided.
When a related art color image forming apparatus for forming a
color image by superimposing a plurality of color images issued,
the defective operation, including the magnification error or the
misregistration of the writing start position, may be generated in
a color basis. Thereby, the main scanning magnification or writing
start position for each color may need to be corrected.
[0008] JP 2003-279873A proposes to an image forming apparatus that
comprises a light source to illuminate in response to image data, a
deflector to deflect an optical beam output from the light source
into a scanning beam running along a main scanning line across an
image forming area in a main scanning direction, first and second
beam detectors detecting the scanning beam at positions in
front-end and rear-end sides at scanning line in the main scanning
direction, a clock generator to generate writing clock signals for
controlling an illumination of the light source, a measurement
mechanism to measure period during a scanning period between
detections of the scanning beam by the first and second beam
detectors, a phase corrector to correct a phase of the writing
clock signals based on correct amount that is set from measurement
result.
[0009] JP2003-323085A proposes an image forming apparatus that
comprises a plurality of beam detectors detecting the scanning beam
at positions at scanning line in the main scanning direction, a
clock generator to generate writing clock signals for controlling
an illumination of the light source, a measurement mechanism to
measure a count number of the writing clock signals generated
during a scanning period between detections of the scanning beam by
one beam detector and another beam detector by predetermined clock
number (two position measurement), a main scanning magnification
corrector to correct a main scanning magnification by the
measurement mechanism and predetermined clock number.
[0010] But if an image forming apparatus uses the above phase
correction technique and two position measurements for correcting a
main scanning magnification, the image forming apparatus cannot
correct a main scanning magnification strictly. This is because
phase correction coverage is between front-end synchronous
detecting position and rear-end image position. So the image
forming apparatus cannot apply phase correction technique to
between rear-end image position and rear-end synchronous detecting
position, and the image forming apparatus has to use two different
frequencies of writing clock signals. And the image forming
apparatus counts writing clock signals between front-end
synchronous detecting position and rear-end synchronous detecting
position for correcting a main scanning magnification. So the
frequency of writing clock signal changes while the image forming
apparatus counts writing clock signals, the image forming apparatus
cannot correct a main scanning magnification strictly.
SUMMARY OF THE INVENTION
[0011] The present invention provides an image reading apparatus
which can correct a main scanning magnification, and the image
forming apparatus controls a frequency of writing clock signals by
counting the writing clock signals using a two position measurement
technique.
[0012] An image forming apparatus according to the invention
includes a light source, a deflector such as a rotating polygonal
mirror, and front and rear beam detectors. Further, there is a
clock generator which generates clock signals to control an
illumination of a light source. There is a phase variable control
unit which varies a phase of the writing clock signal. A
measurement mechanism measures the scanning period between
detections by the two beam detectors. A frequency corrector
corrects a frequency of the writing clock signal so that a count
number measured by the measurement mechanism becomes substantially
equivalent to a predetermined referenced count number. A scanning
period corrector corrects the scanning period at a scanning area
where phase variable control is not available.
[0013] The invention also includes a method of correcting a
scanning period.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] FIG. 1 is a schematic diagram illustrating an image forming
apparatus according to an exemplary embodiment of the present
invention;
[0015] FIG. 2 is a cross sectional view illustrating an optical
device included in the image forming apparatus of FIG. 1;
[0016] FIG. 3 is a schematic top view illustrating the optical
device of FIG. 2;
[0017] FIG. 4 is another schematic top view illustrating the
optical device of FIG. 2;
[0018] FIG. 5 is a block diagram illustrating a configuration of
correcting a main scanning magnification by the optical device of
FIG. 2;
[0019] FIG. 6 is a block diagram illustrating a configuration of a
writing clock generating unit included in FIG. 5;
[0020] FIG. 7 is a flow chart showing the operation of the
invention; and
[0021] FIG. 8 is a timing diagram showing the operation of the
invention.
DETAILED DESCRIPTION OF THE INVENTION
[0022] In describing exemplary embodiments illustrated in the
drawings, specific terminology is employed for the sake of clarity.
However, the disclosure of this patent specification is not
intended to be limited to the specific terminology so selected and
it is to be understood that each specific element includes all
technical equivalents that operate in a similar manner. Referring
now to the drawings, wherein like reference numerals designate
identical or corresponding parts throughout the several views, an
image forming apparatus according to at least a first exemplary
embodiment of the present invention is described.
[0023] Referring to FIG. 1, the image forming apparatus forming
toner images of four colors, black, yellow, cyan, and magenta
includes an optical device 1, photoconductor drums 2K, 2Y, 2C, and
2M, an intermediate transfer belt 3, intermediate transfer rollers
4, development devices 5K, 5Y, 5C, and 5M, a belt cleaning device
6, a transfer device 7, a paper-feed registration sensor 8a, a
registration roller 8b, a fixing device 9, and an ejection device
10. For a full color image forming apparatus, black, yellow, cyan,
and magenta toner colors are respectively indicated using the
suffixes K, Y, C, and M, and these color symbols may be omitted, as
desired.
[0024] The image forming apparatus including the optical device 1
according to at least the first exemplary embodiment of the present
invention employs a light source, for example, a laser diode, to
irradiate surfaces of the photoconductor drums 2K, 2Y, 2C, and 2M
with optical beams so as to form electrostatic latent images. This
exemplary embodiment illustrates a full color image forming
apparatus which forms a full color image by superimposing toner
images of four colors. However, the invention is also applicable to
a monochrome image forming apparatus.
[0025] The optical device 1 emits the laser beams to expose the
surfaces of the photoconductor drums 2K, 2Y, 2C, and 2M. The
photoconductor drum 2K, 2Y, 2C, and 2M form electrostatic latent
images thereon by the laser beams emitted from the optical device
1. The intermediate transfer belt 3 is an intermediate transfer
member on which a toner image is transferred. The intermediate
transfer rollers 4 rotate the intermediate transfer belt 3. The
development devices 5K, 5Y, 5C, and 5M develop the electrostatic
latent images on the photoconductors 2K, 2Y, 2C, and 2M with
toners. The belt cleaning device 6 removes remaining toner from the
intermediate transfer belt 3. The transfer device 7 transfers the
toner image on the intermediate transfer belt 3 onto the transfer
sheet. The paper-feed registration sensor 8a detects a leading end
of the transfer sheet. The registration roller 8b registers the
transfer sheet. The fixing device 9 fixes the toner image on the
transfer sheet. The ejection device 10 ejects the transfer sheet on
which the toner image is fixed.
[0026] The optical device 1 emits the optical beams at controlled
timings so as to expose the photoconductors 2 when an image forming
operation is requested from an operation unit (not shown) by a user
or when a print start signal to start a print job is received from
a host computer connected by a network or other type of wired or
wireless connection.
[0027] A detailed description of the optical device 1 is provided
with reference to FIG. 2 though FIG. 5. Each of the development
devices 5K, 5Y, 5C, and 5M forms a single color image on each of
the respective photoconductor drums 2K, 2Y, 2C, and 2M which is
rotated and exposed to the optical beam. When the photoconductor
drums 2K, 2Y, 2C, and 2M have respective single color toner images
formed thereon, the intermediate transfer belt 3 is rotationally
driven by one of three intermediate transfer rollers 4, for
example, which is a drive roller. The belt 3 rotates in a direction
B shown with an arrow in FIG. 3 around two other intermediate
transfer rollers 4 which are driven rollers. The single color
images formed on the photoconductor drums 2K, 2Y, 2C, and 2M are
sequentially transferred and superimposed onto the intermediate
transfer belt 3 so that a full color image is formed on the
intermediate transfer belt 3.
[0028] When the print start signal is received, a paper-feed unit
(not shown) separates one transfer sheet from a plurality of the
transfer sheets so as to convey the one transfer sheet to the
paper-feed registration sensor 8a. When the one transfer sheet
abuts, contacts, or is proximate to the paper-feed registration
sensor 8a, conveyance of the transfer sheet is stopped. The
registration roller 8b is rotated at a desired timing so that the
transfer sheet is fed between the intermediate transfer belt 3 and
the transfer device 7. Thereby, the full color image is transferred
onto the transfer sheet by the transfer device 7.
[0029] The transfer sheet on which the toner image is transferred
by the transfer device 7 is conveyed to the fixing device 9 where
heat and pressure are applied to fix the transferred image. The
transfer sheet is ejected by an ejection roller included in the
ejection device 10, and is stacked on an ejection tray (not shown).
The image forming apparatus of the exemplary embodiment forms the
images of four colors by employing one optical device that is the
optical device 1 while a related art optical device included in the
related art image forming apparatus has four optical devices to
form the images of four colors. A detailed description of the
optical device 1 included in the image forming apparatus of FIG. 3
is provided with respect to FIG. 2 through FIG. 5. Referring to
FIG. 2, the optical device 1 includes a polygon mirror 20, f.theta.
lenses 21a and 21b, first mirrors 22K, 22Y, 22C, and 22M, curved
axis toroidal lenses 23K, 23Y, 23C, and 23M, second mirrors 24K,
24Y, 24C, and 24M, and third mirrors 25K, 25Y, 25C, and 25M. The
polygon mirror 20 is disposed in a center of the optical device 1.
This polygon mirror 20 deflects the optical beams used for the four
colors in a main scanning direction.
[0030] Optical elements, for example, the fe lenses 21a and 21b,
the first mirrors 22K, 22Y, 22C, and 22M, the curved axis toroidal
lenses 23K, 23Y, 23C, and 23M, the second mirrors 24K, 24Y, 24C,
and 24M, and the third mirrors 25K, 25Y, 25C, and 25M, are disposed
symmetrically to the central polygon mirror 20. Because of this
symmetrical disposition, optical paths for the optical beams of two
colors are provided symmetrically so that the polygon mirror 20
deflects the optical beams of the four colors. As shown in FIG. 3,
the optical paths for black and yellow are provided at a left side
of the polygon mirror 20, and the optical paths for cyan and
magenta are provided at a right side of the polygon mirror 20.
[0031] An operation of the optical device 1 will be given as
follows. Laser diodes mounted in laser units 26K, 26Y, 26C, and 26M
(shown in FIG. 2) emit the optical beams towards cylindrical lenses
27K, 27Y, 27C, and 27M (shown in FIG. 2). The cylindrical lenses
27K, 27Y, 27C, and 27M have respective desired refractive indexes
in a sub-scanning direction so that the optical beams emitted from
the laser units 26K, 26Y, 26C, and 26M are condensed in the
sub-scanning direction, and are directed to a mirror side of the
polygon mirror 20. The polygon mirror 20 deflects the optical beams
in the main scanning direction by high-speed rotations driven by a
motor. The f.theta. lenses 21a and 21b are lenses to correct a
scanning velocity of the laser beams. The first mirrors 22K, 22Y,
22C, and 22M reflect the optical beams deflected by the polygon
mirror 20.
[0032] After the optical beams reflected by the first mirrors 22K,
22Y, 22C, and 22M are directed to the curved axis toroidal lenses
23K, 23Y, 23C, and 23M, the optical beams are directed to the
second mirrors 24K, 24Y, 24C, and 24M. The curved axis toroidal
lenses 23K, 23Y, 23C, and 23M correct a property of the optical
face angle error of the polygon mirror 20. The optical beams
reflected by the second mirrors 24K, 24Y, 24C, and 24M are
reflected by the third mirrors 25K, 25Y, 25C, and 25M so that the
optical beams exit from the optical device 1 in order to form
electrostatic images on the respective photoconductors 2K, 2Y, 2C,
and 2M. As stated above, the optical elements are disposed
symmetrically to the central polygon mirror 20, and the optical
paths for the optical beams of two colors are provided
symmetrically in the optical device 1 of the exemplary
embodiment.
[0033] Referring to FIG. 3, a top view of the optical device 1
includes the polygon mirror 20, the f.theta. lenses 21a and 21b,
the first mirrors 22K, 22Y, 22C, and 22M, the laser units 26K, 26Y,
26C, and 26M, the cylindrical lenses 27K, 27Y, 27C, and 27M, and
reflection mirrors 28a and 28b.
[0034] As stated above, the laser units 26K, 26Y, 26C, and 26M emit
the optical beams from the laser diodes (not shown) so that the
optical beams are directed towards the cylindrical lenses 27K, 27Y,
27C, and 27M. The cylindrical lenses 27K, 27Y, 27C, and 27M have
respective desired refractive indexes in the sub-scanning direction
so that the optical beams emitted from the laser units 26K, 26Y,
26C, and 26M are condensed in the sub-scanning direction. The
reflection mirrors 28a and 28b may be used to reflect the optical
beams so that the optical beams are directed towards the polygon
mirror. When the polygon mirror 20 deflects the optical beams in
the main scanning direction, the first mirrors 22K, 22Y, 22C, and
22M reflect the optical beams deflected by the polygon mirror 20
through the f.theta. lenses 21a and 21b.
[0035] Referring to FIG. 4, another schematic top view of the
optical device 1 is illustrated. The optical beams reflected in
certain positions of the main scanning direction by the second
mirrors 24K, 24Y, 24C, and 24M (shown in FIG. 4) are reflected by
synchronous detection reflection mirrors 29K, 29Y, 29C, and 29M
towards synchronous detection lenses 30a and 30b. Thereby, the
optical beams are reflected to synchronous detection sensors 31a
and 31b. The synchronous detection lenses 30a and 30b condense the
optical beams to the synchronous detection sensors 31a and 31b. The
synchronous detection sensors 31a and 31b are disposed
symmetrically, and detect timings at which the optical beams of two
colors are entered. In other words, the synchronous detection
sensor 31a detects main scanning reference positions of cyan and
magenta while the synchronous detection sensor 31b detects the
black and yellow optical beams.
[0036] Referring to FIG. 5, a configuration of correcting a main
scanning magnification in the optical device is illustrated using
the laser unit 26K as an example. As other laser units 26Y, 26C,
and 26M are configured to be the same as the laser unit 26K,
explanations for these laser units 26Y, 26C, and 26M are
omitted.
[0037] The optical beams emitted from the laser unit 26K are
deflected by the rotations of the polygon mirror 20. As shown in
FIG. 7, the deflected optical beams are received by the synchronous
detection sensor 31b which is disposed outside an image area,
expose the photoconductor drum 2Y, and are received by a rear-end
synchronous detection sensor 61 which is disposed outside the image
area through the f.theta. lens 21b in a sequential manner.
[0038] When the synchronous detection sensor 31b and the rear-end
synchronous sensor 61 receive the optical beams, these sensors
respectively output to a writing clock generating unit 62. This
writing clock generating unit 62 determines or counts a number of
clock signals generated between a time the optical beam is received
by the synchronous detection sensor 31b and a subsequent time the
optical beam is received by the rear-end synchronous detection
sensor 61, using the detection signals DETP_N and EDETP_N. The
writing clock generating unit 62 stores a reference count number.
This reference count number is measured or determined when the main
scanning magnification is in an appropriate state. The writing
clock generating unit 62 compares the measured count number and the
reference count number, and corrects a writing clock frequency such
that the measured count number is substantially equal to the
reference count number (e.g., within 5%, 3%, 1%, 0.5%, 0.1%, or
less, for example). The writing clock generating unit 62 outputs a
writing clock signal CLKO based on the corrected writing clock
frequency. The writing clock generating unit 62 outputs a plurality
of clock signals as the writing clock signals CLKO, each of which
has a different phase. As the writing clock generating unit 62
corrects the main scanning magnification by generating the writing
clock signal, the writing clock generating unit 62 may be referred
to as a magnification correction unit.
[0039] The writing clock signals CLKO output by the writing clock
generating unit 62 are input to a phase synchronous unit 63. The
detection signal DETP_N output by the synchronous detection sensor
31b for every scanning of the optical beam is also input to the
phase synchronous unit 63.
[0040] Among the plurality of writing clock signals CLKO, the phase
synchronous unit 63 selects one having a phase closest to a
synchronous signal by comparing DETP_N with the writing clock
signals CLKO. The clock signal selected by the phase synchronous
unit 63 is output to a LD (laser diode) driver 55 as a writing
clock signal CLK. The LD driver 55 causes the laser unit 26 to emit
based on an image signal (referred to as an image data) and output
the optical beam at a desired timing based on a synchronization to
the writing clock signal CLK.
[0041] Referring to FIG. 6, a configuration of the writing clock
generating unit 62 included in FIG. 5 includes a counter 71, a
control unit 72, and a clock generating unit 73. The operation of
these components is set forth with respect to FIG. 7. Referring to
FIG. 7, when the detection signal DETP_N is input, the counter 71
begins to count a measurement clock signal ICLK. A count number of
the measurement clock signal ICLK from when the detection signal
DETP_N is input until the detection signal EDETP_N is input, is
output to the control unit 72 (step 1). The counter 71 is cleared
by the detection signal DETP_N. A count number output from the
counter 71 represents a scanning period between the synchronous
detection sensor 31b and the rear-end synchronous detection sensor
61
[0042] It is preferable to use writing clock signal CLK outputted
from the phase synchronous unit 63 as a measurement clock signal
ICLK. Because a signal that is synchronous with main scanning
direction cycle makes the counter 71 reset, there is a reduced risk
of miscount due to a phase difference between the synchronous
detection sensor 31b and the rear-end synchronous detection sensor
61. Moreover, the use of CLK as ICLK enables accurate counting of
the clock signal between the synchronous detection sensor 31b and
the rear-end synchronous detection sensor 61 by using the writing
signal CLK that is substantially synchronous with synchronous
detection signal of main scanning direction.
[0043] The control unit 72 calculates the frequency
(f=f.times.Rref/N) that corrects the writing clock frequency based
on the count number N which is measured, etected, or determined,
and the reference count number Rref and outputs data to the clock
generating unit 73 (step 2). The reference count number Rref is a
predetermined number. It is preferable that the count number, which
is measured when precorrecting writing clock frequency operation
occurs, is used as reference count number Rref, and a measurement
clock signal, which is set when correcting writing clock frequency
operation occurs, is used. And it is preferable that the reference
count number Rref, which is referenced by the writing clock
generating unit 62 when the writing clock generating unit 62
controls frequency, is measured when primary correcting writing
clock frequency operation occurs. This is because the reference
count number Rref enables a reduction in influence of variation in
the optical device and/or properties of optical elements and
corrects magnification correctly.
[0044] The control unit 72 corrects writing clock frequency based
on the count number N which is measured and the reference count
number Rref. If a writing clock frequency is f, and corrected
writing clock frequency is F', the control unit 72 can get
corrected writing clock frequency by using below Equation 1.
f'=f.times.Rref/N Equation 1
[0045] The control unit 72 calculates the frequency based on the
count number N which is measured between synchronous detection
sensors 31b and a rear-end synchronous detection sensor 61 and
which is substantially equivalent to a predetermined reference
count number Rref.
[0046] The clock generating unit 73 generates clock signals CLKO
which corresponds to f' from the control unit 72 and outputs
generated clock signals CLKO to the phase synchronous unit 63 (step
3). Main scanning magnification correction for Yellow, Cyan, and
Magenta is the same as what has been described above for Black.
[0047] A PLL (phase locked loop) circuit is used for clock
generating. But, the frequency generated by a PLL circuit is
dispersed, so phase variable control that varies a phase of writing
clock signals in units of 1/n of a writing clock cycle, where n is
an integer of 2 or greater, at a position or a plurality of
positions in the main scanning direction, may be used for slight
adjustment of the clock generated by the PLL circuit.
TABLE-US-00001 It is preferable that the above slight adjustment
amount (phase variable amount) is listed and stored as de- scribed
below. The slight adjustment Phase variable amount at amount is
determined by the target area where phase variable writing clock
frequency control is available f1 DN1 f2 DN2 f3 DN3 f4 DN4 f5 DN5
f6 DN6 f7 DN7 f8 DN8 f9 DN9 F10 DN10 F11 DN11 F12 DN12 F13 DN13 F14
DN14 F15 DN15 F16 DN16 F17 DN17 F18 DN18 F19 DN19 F20 DN20 .cndot.
.cndot. .cndot. .cndot. .cndot. .cndot. .cndot. writing clock
frequency
[0048] FIG. 8 shows there is an area where phase variable control
is available and there is area where phase variable control is not
available. If the image forming apparatus generates the writing
clock signal using a PLL circuit and phase variable control, the
counter 71 counts a measurement clock signal between detection
signals DETP_N outputted by the synchronous detection sensor 31b
and detection signals EDETP_N outputted by rear-end synchronous
detection sensor 61, by writing clock frequency: fpd generated by
the PLL circuit and phase variable control at an area where phase
correcting technique is available. The counter 71 also counts a
measurement clock signal by writing clock frequency: fp generated
by PLL at area where phase variable control is not available. The
counter 71 counts a measurement clock signal using a different
clock frequency at an area where phase variable control is
available and an area where phase variable control is not
available, so the image forming apparatus may not be able to
completely correct the main scanning magnification.
[0049] The invention includes an image forming apparatus as
described in the below three embodiments which can properly correct
the main scanning magnification.
Embodiment 1
[0050] The control unit 72 stores a list or table as set forth
below. The table has a phase variable amount DN' at an area where
phase variable control is not available. The phase variable amount
DN' is a predetermined ideal amount.
TABLE-US-00002 Writing clock Phase variable amount at Phase
variable amount at signal area where phase variable area where
phase variable frequency control is available control is not
available f1 DN1 DN'1 f2 DN2 DN'2 f3 DN3 DN'3 f4 DN4 DN'4 f5 DN5
DN'5 f6 DN6 DN'6 f7 DN7 DN'7 f8 DN8 DN'8 f9 DN9 DN'9 f10 DN10 DN'10
f11 DN11 DN'11 f12 DN12 DN'12 f13 DN13 DN'13 f14 DN14 DN'14 f15
DN15 DN'15 f16 DN16 DN'16 f17 DN17 DN'17 f18 DN18 DN'18 f19 DN19
DN'19 f20 DN20 DN'20 .cndot. .cndot. .cndot. .cndot. .cndot.
.cndot. .cndot. .cndot. .cndot.
[0051] The control unit 72 references the table to read the phase
variable amount DN' in accordance with the count number N and
writing clock frequency fn that is measured as described above. The
control unit 72 corrects the count number N based on phase variable
amount DN'. If the corrected count number is N', the coefficient
for converting the phase variable amount DNn' into count number is
.alpha., the control unit 72 can get the corrected count number N'
using Equation 2.
N'=N+.alpha..times.DNn' Equation 2
[0052] The control unit 72 can determine the corrected writing
clock frequency using equation 1 and by using N'. Thus, the image
forming apparatus can correct the main scanning magnification
correctly. Because the control unit 72 can get the count number
that is substantially the same count number that is measured by
almost the same frequency between the detection signals DETP_N
outputted by the synchronous detection sensor 31b and the detection
signals EDETP_N the outputted by the rear-end synchronous detection
sensor 61.
[0053] For example, if N is 20000[ 1/16 dot], DNn' is 10[1/4 dot],
.alpha. is 4, fn is 50.0 [MHz], Rref is 20200[ 1/16 dot], the
control unit 72 can get corrected writing clock frequency f using
Equation 3.
f ' = fn .times. Rref / N = f .times. Rref / ( N + .alpha. .times.
DNn ' ) = 50.0 .times. 20200 / ( 20000 + 4 .times. 10 ) = 50.3992 [
MHz ] Equation 3 ##EQU00001##
[0054] In this embodiment, the control unit 72 has a phase variable
amount DN'. The control unit 72 can store values considered a
instead of the phase variable amount DN'.
Embodiment 2
[0055] The control unit 72 calculates phase variable amount DN' in
accordance with the area where phase variable control is not
available using DNn that is a phase variable amount at an area
where phase variable control is available. If T1 is a scanning
period at an area where phase variable control is available, and T2
is a scanning period at an area where a phase variable control is
not available, the control unit 72 can calculate the phase variable
amount DN' using Equation 4.
DNn'=DNn.times.T2/T1 Equation 4
[0056] The control unit 72 corrects the count number N by adding a
result of DNn.times.T2/T1, if the writing clock frequency is fn and
the coefficient for converting the phase variable amount DNn' into
count number is .alpha..
[0057] The control unit 72 can get the corrected count number N'
using Equation 5.
N'=N+.alpha..times.DNn.times.T2/T1 Equation 5
[0058] The control unit 72 can get the corrected writing clock
frequency using the above equation 1 by using N'. So, the image
forming apparatus can correct the main scanning magnification
correctly. Because the control unit 72 can get the count number
that is substantially the same count number that is measured by
almost same frequency between detection signals DETP_N outputted by
the synchronous detection sensor 31b and detection signals EDETP_N
outputted by rear-end synchronous detection sensor 61.
[0059] For example, if N is 20000[ 1/16 dot], DNn is 200[1/4 dot],
.alpha. is 4, fn is 50.0 [MHz], Rref is 20200[ 1/16 dot], T1 is 300
[.mu.s], T2 is 30 [.mu.s], the control unit 72 can get the
corrected writing clock frequency f using Equation 6.
f ' = fn .times. Rref / N = f .times. Rref / ( N + .alpha. .times.
. DNn .times. T 2 / T 1 ) = 50.0 .times. 20200 / ( 20000 + 4
.times. 200 .times. 30 / 300 ) = 50.2988 [ MHz ] Equation 6
##EQU00002##
Embodiment 3
[0060] The control unit 72 can correct the count number N by using
fpd generated by the PLL circuit and phase variable control at an
area where the phase correcting technique is available, and fp
generated by PLL at an area where the phase variable control is not
available.
[0061] The control unit 72 can get the corrected count number N'
using Equation 7.
N'=N+T2.times.(fpd-fp) Equation 7
[0062] The control unit 72 can get the corrected writing clock
frequency using the above equation 1 by using N'. So, the image
forming apparatus can correct main scanning magnification
correctly. The control unit 72 can get the count number that is
substantially the same count number that is measured by almost the
same frequency between detection signals DETP_N outputted by the
synchronous detection sensor 31b and detection signals EDETP_N
outputted by the rear-end synchronous detection sensor 61.
[0063] For example, if N is 20000[ 1/16 dot], DNn is 200[1/4 dot],
.beta. is 16, Rref is 20200[ 1/16 dot], T2 is 30 [.mu.s], fdp is
50.0 [MHz], fp is 49.0 [MHz], the control unit 72 can get the
corrected writing clock frequency f using Equation 8.
f ' = fpd .times. Rref / N = f .times. Rref / ( N + .beta. .times.
T 2 .times. ( fpd - fp ) ) = 50.0 .times. 20200 / = ( 20000 + 16
.times. 30 .times. 10 - 6 .times. ( 50.0 - 49.9 ) .times. 10 6 ) =
50.2988 [ MHz ] Equation 8 ##EQU00003##
[0064] The main scanning magnification correction for Yellow, Cyan,
and Magenta is the same as the above mentioned main scanning
magnification correction for Black.
[0065] This invention is applicable to mono-color image forming
apparatuses, and also plural color image forming apparatuses.
[0066] Obviously, numerous additional modifications and variations
of the present invention are possible in light of the above
teachings. It is therefore to be understood that within the scope
of the appended claims, the present invention may be practiced
otherwise than as specifically described herein.
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